Speaker Cable VS Instrument Cable

[Lifeofsobriety]

So this is kind of a noob question.

What is the difference between a speaker cable and an instrument cable.

Heres the issue:

Me and my buddy got together for a jam the other day. A few days later he realizes his speaker cable is missing. I look through my cables and alas there is an extra one there. The issue is the look exactly the same. A generic black cable with silver ends.... We arn't sure how to tell the difference. If we were to guess the wrong cable and use the instrument cable to go from his amp to his cab could it damage the amp? and is there any way to really tell the difference?

Thanks

Dan